"I think it underscores the threat that al Qaeda in the Arab Peninsula poses to U.S. interests," President Obama's top counterterrorism adviser John Brennan told ABC's "This Week."

"We looked at the intelligence that is available as far as the plans for al Qaeda to carry out attacks in Sanaa possibly against our embassy, possibly against U.S. personnel, and decided it was the prudent thing to do to shut the embassy," Brennan said.

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The British government joined the United States in closing its embassy in Yemen on Sunday, highlighting the Arab nation's emergence as one of the world's premier terrorist havens.

Yemen: New Front in War on Terror?

In the wake of the Christmas Day terror attempt, the president's deputy national security adviser for homeland security and counterterrorism reiterated the Obama's administration has been "investing in Yemen for many, many months."

"Al Qaeda has several hundred members in Yemen, and they've grown in strength," Brennan said. "From the very first day of this administration we've been focused on Yemen."

"Just this past month, we and the Yemenis were able to identify the location of some of these al Qaeda operatives and commanders and leaders, successful strikes that were carried out, and there were several of the al Qaeda members, operatives and the senior leaders who are no longer with us today as a result of those actions," Brennan said.

Congresswoman: Stop Sending Guantanamo Bay Detainees to Yemen

Rep. Jane Harman, chair of the House Homeland Security Intelligence Subcommittee, argued Guantanamo Bay detainees should not be sent back to Yemen in light of the country's struggle to contain al Qaeda.

"I believe the prison should close, but I also believe we should review again where we're going to send the detainees. I think it is a bad time to send the 90 or so Yemenis back to Yemen," Harman, D-Calif., said on "This Week."

The top Republican on the House Intelligence Committee argued on "This Week" that former Guantanamo Bay detainees are making up the "core group" of al Qaeda in the Arab Peninsula.

"This is an imminent threat that is coming from the Al Qaeda Arabian Peninsula area," said Rep. Peter Hoekstra, R-Mich., in his first interview since visiting Yemen.

"The core group of al Qaeda on the Arabian Peninsula is formed by former Gitmo detainees," Hoekstra said. "These are people that were held in Gitmo, have been returned and have now gone back to the battlefield."

Hoekstra said the other element posing a threat in Yemen is radical cleric Anwar al-Awlaki.

"So you've put the Gitmo folks together. You put Awlaki together. These people have moved an attack on the U.S. homeland to their -- to the top of their priority list. So that is the root cause of why we saw the attack at Fort Hood, why we saw the attack on the flight, Flight 253," Hoekstra said.

On "This Week" Hoekstra defended a fundraising letter he sent to his supporters this week asking for money citing the Flight 253 terror plot, citing the administration's "refusal to acknowledge that the Fort Hood attack was a terrorist attack."

"I've been right on the facts all along on this -- on the recent attacks, the connections with Yemen," he said. "I've been trying to drive this administration in a policy direction that keeps America safe."

Counterterror Official: No 'Smoking Gun' on Abdulmutallab

Just like Umar Farouk Abdulmutallab, the 23-year-old Nigerian charged with trying to blow up Flight 253, was known to the CIA and had been on a U.S. watch list before the Northwest flight, there have been other instances where the government overlooked terror threats.

Before the June 2009 Little Rock Army recruiting booth fatal shooting of one soldier and the critical injury of another, Muslim convert Abdulhakim Muhammed had been under investigation by the FBI's Joint Terrorist Task Force since his return from Yemen.

Similarly, Maj. Nidal Malik Hasan was the subject of a federal inquiry before the Fort Hood shootings.

After the failed terror attack against Flight 253 Christmas Day, the president ordered reports from the CIA, the Department of Homeland Security and other agencies.

But when asked on "This Week" where the system failed, the president's top counterterrorism official refused to pinpoint one government agency.

"There was no single piece of intelligence, a smoking gun, if you will, that said that Mr. Abdulmutallab was going to carry out his attack against that aircraft," Brennan said, "What we had, looking back at it now, were a number of streams of information."

Brennan said the U.S. government had "snippets" of intelligence about Abdulmutallab, but nothing connecting all of the information together.

"We may have had a partial name, we might have had an indication of a Nigerian, but there was nothing that brought it all together," Brennan said.

Brennan, one of the architects of the National Counterterrorism Center, argued cross-agency intelligence sharing has gotten better since the Sept. 11 attacks, but admitted more work needs to be done in stitching intelligence together.

"All of the information was shared, except that there are millions upon millions of bits of data that come in on a regular basis. What we need to do is make sure the system is robust enough that we can bring that information to the surface that really is a threat concern," he said. "We need to make the system stronger. That's what the president is determined to do."

Asked why the U.S. intelligence community doesn't seem to have to the connection ability of Facebook, with its 350 million users who put out 3.5 billion pieces of content a week, Brennan said: "Well, in fact, we do have the sophistication and power of Facebook and well beyond that. That's why we're able to stop Mr. Najibullah Zazi, David Headley, other individuals from carrying out attacks -- because we were able to do that on a regular basis."

But, he added, "In this one instance, the system didn't work. There were some human errors. There were some lapses. We need to strengthen it. But day in and day out, the successes are there."

Independent Sen. Joe Lieberman, who has long opposed the closing of Guantanamo Bay, argued a military commission should deal with Abdulmutallab.

"That was an act of war. He should be treated as a prisoner of war. He should be held in a military brig, Lieberman said. "He should be questioned now and should have been ever since he was apprehended for intelligence that could help us stop the next attack or get the people in Yemen who directed him to do what he did, so, yes, we -- we should follow the rule of law, but the rule of law that is relevant here is the rule of the law of war."

Administration Official Defends Napolitano

Brennan defended Homeland Security Secretary Janet Napolitano who last week initially assured the public that security systems are working as designed, saying "the system works."

"I think Secretary Napolitano clarified her remarks about the system working or not. I have been able to work with Secretary Napolitano the last 11 months, and I consider that we as a nation are very fortunate to have somebody of Secretary Napolitano's caliber, experience, and dedication," he said.

The top Republican on the Senate Homeland Security Committee characterized Napolitano's initial comments after the Christmas Day terror attempt as "bizarre."

"I will say that her initial comments were bizarre and inappropriate. It baffled me that she said that the system worked very, very smoothly, when clearly it did not. It also surprised me when she implied that there was not information to indicate that this individual posed a threat when there was information," said Sen. Susan Collins, R-Maine, today on "This Week."

But Collins said Napolitano continues to have her support as Secretary of Homeland Security.

Lieberman said the Senate Homeland Security Committee will begin "a series of hearings" when Congress returns in a few weeks to examine "where we are five years after the 9/11 Commission reforms went into effect, seven years after the Department of Homeland Security went into effect."

"We're not out to protect anybody or attack anybody," Lieberman said on "This Week."

"We're out to fix what went wrong on Dec. 25 and to note that, in this year, last year, 2009, our homeland -- there were -- there were more than a dozen attempts to attack our homeland, and three of them broke through," he said.

"So it's time to take a fresh, nonpartisan look, not to knock down the Department of Homeland Security or the 9/11 reforms, but, frankly, to fix and build them up so we learn from our mistakes and we're more secure in the future," Lieberman said.